Nanotechnology Spotlight

Superhydrophobic tracks for drop transport open path to low-cost microfluidics

(Nanowerk Spotlight) The primary measurement to determine wettability – the actual process when a liquid spreads on a solid substrate or material – is the angle between the solid surface and the surface of a liquid droplet on the solid's surface. For example, a droplet of water on a hydrophobic surface would have a high contact angle, but a liquid spread out on a hydrophilic surface would have a small one. Surfaces where the contact angle is approaching 180° are called superhydrophobic and surfaces where the contact angle is approaching 0° are called superhydrophilic. Advanced material engineering techniques can structure surfaces that allow dynamic tuning of their wettability all the way from superhydrophobic behavior to almost complete wetting (see: "'Nailing' superlyophobic surfaces with nanotechnology").

Researchers in Finland and Israel have now explored the feasibility of using superhydrophobicity for guided transport of water droplets in microfluidic devices. Reporting their work in a recent issue of Advanced Materials ("Superhydrophobic Tracks for Low-Friction, Guided Transport of Water Droplets"), they demonstrate a new, simple and general approach for transportation of water droplets based on superhydrophobic technology. Water droplets are transported at high velocity in almost totally water-repellent tracks with vertical walls. Drops move in open tracks, machined in metal or silicon wafers, using gravity or using electrostatic charge.

"Usually, in digital microfluidics, droplets are formed in a system of two immiscible liquids, for example, water in oil, and the liquid volume is transported in microchannels using pumps and valves," Henrikki Mertaniemi, a student in Aalto University's Molecular Materials group, tells Nanowerk. "In our approach, droplets in air can be used. Employing superhydrophobic tracks, droplets can be transported with low friction and high efficiency."

Electrostatic actuation of a water droplet in a superhydrophobic track. A charged glass rod is employed for providing the electrostatic force for moving a water drop in a superhydrophobic track on a horizontal surface. Real time.

Digital microfluidics is a relatively new field in microfluidics that studies controlling and analyzing single liquid droplets instead a continuous flow of liquids in channels. The research requires a multidisciplinary approach, and thus, it has been performed in cooperation and interaction between physicists and chemists from Aalto University, University of Helsinki (both Finland) and Technion (Israel).

Using a suitable superhydrophobic track on a tilted plate, no external power is needed for droplet transport, since gravity can be used for providing the required force. Thus, transport can be made very efficient. Furthermore, drops can move quite rapidly in the tracks, as shown for example in the video below:

Superhydrophobic track. Water drops passing multiple curves in a superhydrophobic track with a width of 1.5 mm. The plate is tilted only slightly. Real time.

The researchers prepared two different types of superhydrophobic tracks, ones having a bottom and others bottomless, on different substrates. Tracks with a bottom were made by milling of shallow grooves into copper plates.

Tracks without a bottom were cut entirely through the substrate and were made in zinc plates by laser cutting, or to silicon wafers by etching. The tracks on silicon substrates had a width of 500 µm. Mertaniemi says that the team fabricated much smaller tracks, down to 100 µm, but these could not be tested due to difficulties in manually producing small enough droplets.

He notes that tracks with very small widths would be suited for systems with an automatic dispensing system capable of producing sub-microliter droplets.

The metal tracks in copper and zinc were made superhydrophobic by galvanic deposition of silver followed by deposition of a fluorinated thiol surfactant. The silicon tracks were made superhydrophobic by nanograss generated by plasma etching followed by a plasma deposition of fluoropolymer.

The team also demonstrated that droplets can be cut in half using a superhydrophobic blade positioned in the middle of the track, and droplets with different volumes can be separated using a track with a local widening.

"To our knowledge, a superhydrophobic knife was not presented before" says Mertaniemi. "Using the superhydrophobic track to guide the droplet collision with the blade, droplets can be cut accurately and consistently.

Superhydrophobic knife. A water drop is split into two smaller drops by a superhydrophobic knife in a superhydrophobic track. The video is slowed down by a factor of 4.

"Superhydrophobic tracks will probably find their applications in digital microfluidic devices where facile transport of droplets or drop splitting is required, including, for example, biotechnical and medical research equipment" says Robin Ras, an Academy Research Fellow at Aalto University, who led this work. " We foresee that this technology for drop transport could open up new directions to low-cost microfluidic applications that, in contrast to most current drop transport technology, do not rely on electric power. In addition, combining suitable track geometry to a computer-controlled electric field or tilting stage would enable the programming of complex trajectories for droplets."